1. Field of the Invention
The present invention relates to a power semiconductor device of a voltage-controlled transistor, such as power MOS field effect transistor (power MOSFET) and insulating gate bipolar transistor (IGBT).
2. Description of the Prior Art
The voltage-controlled power semiconductor devices include, for example, power MOS field effect transistors and insulating gate bipolar transistors, and each of them is provided with a gate oxide as its essential constituent element. FIG. 12 is a sectional view showing the construction of a power MOS field effect transistor of the prior art, and FIG. 13 shows its equivalent circuit. As illustrated in FIG. 12, the conventional power MOS field effect transistor is, for example, formed as follows: On an n.sup.+ substrate 1 made of n-type silicon, an n.sup.+ epitaxial layer 3 formed by n-type silicon subjected to epitaxial growth, a p-type region 4 formed by diffusing p-type impurities in the n.sup.+ epitaxial layer 3 and n.sup.+ source region 5 formed by diffusing n-type impurities therein are stacked, and on these layers, a gate 7, insulated by a gate oxide 6 made of, for example, oxide silicon, is formed and Al conductor 8 serving as a source electrode is further formed. Moreover, in the power MOS field effect transistor of the prior art as shown in FIG. 12, n-type polysilicon layers 9 and p-type polysilicon layers 10 alternately placed by using polycrystalline silicon (polysilicon) used at the time of forming the gate electrode so that n-number of zener diodes d11, d12, . . . , d1n are formed, thereby preventing a high voltage, which might cause breakdown of the insulation of the gate oxide 6, from being applied to the gate oxide 6.
The power MOS field effect transistor, configured as described above, is allowed to carry out switching operations by turning ON/OFF the gate voltage. In general, since this gate oxide is a comparatively thin film, if there is any defect in the gate oxide, dielectric breakdown tends to occur at the corresponding portion, causing breakdown of the device. For this reason, in general, after devices have been manufactured, a voltage, which is higher than the actually used voltage, is applied between the gate and source, thereby breaking and eliminating (screening) those originally weak devices containing any defect in the gate oxide; thus, only the devices having sufficient reliability can be shipped. Moreover in recent years, there have been demands for devices having a low switching voltage, and in response to these demands, attempts to reduce the switching voltage have been made by making the thickness of the gate oxide thinner or reducing the diffusion concentration (impurity concentration). As the gate oxide is made thinner, the dielectric breakdown voltage of the gate oxide is lowered, with the result that the possibility of breakdown due to defects, etc. occurring upon formation of the gate oxide become higher; therefore, more effective screening is required. Moreover, besides the power MOSFETs, power semiconductor devices of the voltage-control type include insulation gate bipolar transistors (IGBT), and the IGBT is also provided with a gate insulator in the same manner as the power MOSFET and normally with zener diodes, and therefore requires effective screening during its production process.
However, in the voltage-controlled power semiconductor device, since zener diodes are generally formed therein as described above, the voltage used for screening is set as low as possible, while the voltage of protection-use zener diodes has to be set to a voltage higher than the screening voltage. For this reason, in the conventional power semiconductor devices, it is not possible to apply any voltage exceeding a predetermined value to the gate oxide, resulting in failure to carry out an effective screening.
For example, in a power semiconductor device, in the case when zener diodes, which have a voltage-current characteristic as shown in FIG. 14, are formed as a protection circuit, upon receipt of an excessive current such as a surge, the gate oxide is subjected to a voltage application corresponding to the withstanding voltage VBG of the zener diodes plus the value of the voltage drop. Therefore, even in the case of screening by the use of a voltage not more than the withstanding voltage VBG, there is a possibility that the device might be broken by a voltage not more than the withstanding voltage VBG even though the zener diodes are provided.
Moreover, when the withstanding voltage VGB of the zener diodes is set lower, the voltage that can be applied upon screening has to be further lowered, with the result that it becomes more difficult to eliminate the initially defective products.